We are trying to identify novel or known viruses as causes of diseases of uncertain etiology. Blood and tissues from patients are being examined by sensitive PCR tests in an attempt to find new or known viruses that are responsible for the diseases. Previously, we used these procedures to identify human herpesvirus 6 in lymph node biopsies from three patients who presented with fever and enlarged lymph nodes. Cytomegalovirus causes congenital disease which can result in deafness and mental retardation in neonates, and can cause severe viral pneumonia and colitis in transplant recipients and sight-threatening retinitis in patients with AIDS. Epstein-Barr virus (EBV) causes infectious mononucleosis and is associated with a number of cancers including Burkitt lymphoma, nasopharyngeal carcinoma, Hodgkins disease, and post-transplant lymphoproliferative disease. Human CMV and EBV infect humans, but not small animals or nonhuman primates. The best models currently available for CMV and EBV are rhesus monkey CMV and EBV. The goal of this study is to develop an effective vaccine for these rhesus viruses and to use these as a model for vaccines for their human counterparts. We are using various approaches including soluble recombinant proteins, recombinant virus vectors expressing viral proteins, and replication defective viruses as vaccines. Measurement of neutralizing antibodies to EBV is important for evaluation of candidate vaccines. The current neutralization assay is based on antibody inhibition of EBV transformation of primary B cells and requires 6 weeks to perform. This year, we developed a rapid, quantitative flow cytometry assay and show that neutralizing antibody titers measured by the new assay strongly correlate with antibody titers in the standard transformation-based assay. Antibodies to EBV gp350 and gp42 have been shown to block infection of B cells by EBV. Using new assays to quantify antibodies to these glycoproteins, we show for the first time that human plasma contains high titers of antibody to gp42, and that these titers correlate with neutralization of EBV infectivity or transformation. Furthermore, we show that antibody titers to EBV gp350 correlate more strongly with neutralization than antibody titers to gp42. These assays should be useful in accessing antibody responses to candidate EBV vaccines.